Dissociation of CDK2 from cyclin A in response to the topoisomerase II inhibitor etoposide in v-src-transformed but not normal NIH 3T3 cells

P-glycoprotein attenuates DNA repair activity in multidrug-resistant cells by acting through the Cbp-Csk-Src cascade

Recent studies have demonstrated that P-glycoprotein (P-gp) expression impairs DNA interstrand cross-linking agent-induced DNA repair efficiency in multidrug-resistant (MDR) cells. To date, the detailed molecular mechanisms underlying how P-gp interferes with Src activation and subsequent DNA repair activity remain unclear. In this study, we determined that the C-terminal Src kinase-binding protein (Cbp) signaling pathway involved in the negative control of Src activation is enhanced in MDR cells. We also demonstrated that cells that ectopically express P-gp exhibit reduced activation of DNA damage response regulators, such as ATM, Chk2, Braca1 and Nbs1 and hence attenuated DNA double-strand break repair capacity and become more susceptible than vector control cells to DNA interstrand cross-linking (ICL) agents. Moreover, we demonstrated that P-gp can not only interact with Cbp and Src but also enhance the formation of inhibitory C-terminal Src kinase (Csk)-Cbp complexes that reduce phosphorylation of the Src activation residue Y416 and increase phosphorylation of the Src negative regulatory residue Y527. Notably, suppression of Cbp expression in MDR cells restores cisplatin-induced Src activation, improves DNA repair capacity, and increases resistance to ICL agents. Ectopic expression of Cbp attenuates cisplatin-induced Src activation and increases the susceptibility of cells to ICL agents. Together, the current results indicate that P-gp inhibits DNA repair activity by modulating Src activation via Cbp-Csk-Src cascade. These results suggest that DNA ICL agents are likely to have therapeutic potential against MDR cells with P-gp-overexpression.

Susceptibility of Hep3B cells in different phases of cell cycle to tBid

tBid is a pro-apoptotic molecule. Apoptosis inducers usually act in a cell cycle-specific fashion. The aim of this study was to elucidate whether effect of tBid on hepatocellular carcinoma (HCC) Hep3B cells was cell cycle phase specific. We synchronized Hep3B cells at G0/G1, S or G2/M phases by chemicals or flow sorting and tested the susceptibility of the cells to recombinant tBid. Cell viability was measured by MTT assay and apoptosis by TUNEL. The results revealed that tBid primarily targeted the cells at G0/G1 phase of cell cycle, and it also increased the cells at the G2/M phase. 5-Fluorouracil (5-FU), on the other hand, arrested Hep3B cells at the G0/G1 phase, but significantly reduced cells at G2/M phase. The levels of cell cycle-related proteins and caspases were altered in line with the change in the cell cycle. The combination of tBid with 5-FU caused more cells to be apoptotic than either agent alone. Therefore, the complementary effect of tBid and 5-FU on different phases of the cell cycle may explain their synergistric effect on Hep3B cells. The elucidation of the phase-specific effect of tBid points to a possible therapeutic option that combines different phase specific agents to overcome resistance of HCC.

Oncogenic tyrosine kinases and the DNA-damage response

Oncogenic tyrosine kinases (OTKs) are involved in the induction of many types of tumour, including haematological malignancies and cancers of the breast, prostate, colon and lung. Neoplastic cells that express OTKs are usually resistant to apoptosis that is induced by DNA-damaging agents, such as cytostatic drugs and irradiation, and they display genomic instability. So, what are the mechanisms involved, and what is the potential for overcoming OTK-mediated resistance in the clinic?

Overexpression of DA41 in v-Ha-ras-3Y1 cells causes growth suppression

We have recently found that DA41 exhibits marked homology with mouse PLIC-1, PLIC-2, frog XDRP1 and yeast DSK2. XDRP1 has been shown to be associated with cyclin A, and blocks cell division of frog embryo. In the present study, we examined the biological role(s) of DA41 in mammalian cells by overexpressing it in v-Ha-ras-transformed 3Y1 cells (ras-3Y1). Transfectants which expressed a high level of DA41 mRNA exhibited a decrease in growth rate, a reduction in saturation density, and a suppression of colony formation in soft agar medium. To clarify the molecular mechanism(s) by which DA41 affects cell growth, the effect of DA41 expression on the levels of various cell cycle-regulatory proteins was examined. The forced expression of DA41 gene resulted in a remarkable reduction in CDK2 activity, while the amount of CDK2 did not change. These observations indicate that DA41 is involved in cell cycle regulation in ras-3Y1 cells.